A corona discharge arrangement of the kind to which the present invention pertains consists, in principle, of at least one corona electrode and at least one target electrode. The corona and target electrodes are spaced apart, and each of the electrodes is connected to a respective pole of a d.c. source, and the configuration of the corona electrode and the pole voltage of the d.c. source are such as to generate a corona discharge on the corona electrode. This corona discharge results in the generation of electrically charged air-ions of the same polarity as the corona electrode, and the air-ions migrate towards the target electrode, under the influence of the electric field, where they surrender their electric charge.
Corona discharge arrangements of the principle construction are found in many different forms in practice and are used for many different purposes. For example, corona discharge arrangements of this kind are used to a significant extent in air-purifying electrostatic precipitators. In this case, the air ions generated by the corona discharge are operative to charge electrically the contaminants, particles and/or liquid droplets, present in the air in the form of aerosols. These electrically charged impurities are extracted from the air, by being attracted to and bound on collecting surfaces of opposite polarity. Corona discharge arrangements of this kind are also used in air-transportation systems in which air flows are driven with the said of so-called electric ion or corona wind. Various different designs of such air transportation systems are found described in the literature, for instance in international patent applications PCT/SE85/00538, PCT/SE86/00548, PCT/SE87/00595 and PCT/SE88/00365 respectively now U.S. Pat. No. 4,812,711, U.S. Pat. No. 5,006,761, U.S. Pat. No. 5,024,685 and U.S. Ser. No. 07/438,427 now U.S. Pat. No. 5,012,159. Corona discharge arrangements of similar principle construction are also used in various other apparatus, as for instance electrostatic copying machines and laser printers.
A well known and not-readily solved problem encountered with the use of such corona discharge arrangements in spaces or locations in which people are to be found, for instance in dwellings or working places, and also in ventilation systems and air treatment systems which communicate with such spaces or locations, is that the corona discharge will generate chemical compounds, primarily ozone and nitrogen oxides, which are liable to cause an irritating effect and which may even be harmful to people when the compounds are present in excessive quantities. It is also known that a corona discharge will give rise to strong and unpleasant odors, which persist even subsequent to termination of the corona discharge process and subsequent to dispersal of the ozone generated by said process. The nature of these odorous gaseous substances is not known at present, although it is reasonable to assume that they occur as a result of chemical changes in airborne organic compounds in the highly reactive plasma layer of the corona discharge. The odorous substances generated by the discharge may therefore be potentially very harmful.
All of the undesirable, initiating or harmful substances are created in the immediate vicinity of the corona electrode and spread to the ambient air. The substances are produced at a rate which is contingent on the size of the electric corona current, and much higher levels of contaminating substances are produced by a negative corona discharge than by a positive corona discharge. The problem still exists, however, in the case of positive corona discharges. This generation of harmful or irritating substances has seriously limited the possibilities of using corona discharge arrangements in populated environments and has meant that the corona current used with such arrangements must be restricted. This limitation of the size of the corona current used, however, is in conflict with the intended purpose of such corona discharge arrangements, to wit the generation of a large number of electrically charged air-ions.
One solution to this problem is described in international patent application PCT/SE87/00183, according to which the air located in the immediate vicinity of the corona electrode and carrying the undesired, harmful substances, is removed continuously, e.g. by suction, through a tubular channel element which has an inflow orifice located in the immediate vicinity of the corona electrode and through which the contaminated air removed from the vicinity of the electrode is passed either to an air-filtering arrangement, in which the air is cleansed of its contaminant content, e.g. with the aid of sorbents, or alternatively to a location at which the air and the substances carried thereby can be released safely to the surroundings, i.e. to the open air. This solution to the problem in question is correct in principle and can also be applied in practice It is proposed in the aforesaid international patent application that the requisite flow of air passed the corona electrode and into the suction channel is generated with the said of a fan, an air pump or some corresponding device connected to the suction channel. One drawback with this solution, however, is that the provision of a fan, an air pump or the like for this purpose is both cost and space demanding, besides which the presence of such a fan, air pump etc., will add significantly to the noise generated by the system as a whole. It is also suggested in the aforesaid international patent application that the contaminated air could conceivably be guided through the inflow orifice of the suction channel with the said of air which already flows past the corona electrode and target electrode of the corona discharge arrangement, this airflow being sustained, for instance, by an external existing fan or being generated by the ion wind which flows from the corona electrode to the target electrode, and thus obviates the need for connecting a separate fan, air pump or corresponding device to the suction channel. This solution, however, requires the inflow orifice of the suction channel to be located immediately downstream of the corona electrode, as seen in the direction of the prevailing airflow, and also very close to the corona electrode, which in many instances would require an unacceptable structural configuration of the corona discharge arrangement.
Generally speaking, in order to be able to remove through the suction channel a significant proportion of the harmful substances generated by the corona discharge, it is necessary for the airflow carrying the contaminating substances to pass very close to the corona electrode and from there rapidly through the inflow orifice and into said suction channel, since these contaminating substances are generated in the close proximity of the corona electrode and tend to disperse rapidly in the ambient air. In many cases, it is found that this is difficult to achieve in practice at the level of efficiency desired, when practicing the method described in the aforesaid international patent application. It has also been found that the end of the suction channel, or tube, located in the close proximity of the corona electrode has, in many instances, an unfavourable screening and disturbing effect on the corona electrode, such as to make it difficult to produce and sustain the desired corona discharge without increasing the voltage difference between the corona and target electrodes or reducing the distance between these electrodes. Such increases in voltage difference or reductions in the distance between the electrodes, however, are not desirable, since the general endeavour is to produce the largest possible ion flow from corona electrode to target electrode with the said of the lowest possible voltage and with significant spacing between the corona and target electrodes. This latter criterium is particularly important when the corona discharge arrangement is used to drive airflows with the said of so-called ion wind.